New evidence suggests the plague is much older than we thought

Thomson Reuters Plague is perhaps one of the most well-known and deadly diseases of ancient times. The rod-shaped bacteria, Yersinia pestis, can sicken both humans and animals, and is responsible for at least three historic pandemics within the past 1,500 years.

Chief among these was the Black Death of the mid-1300s, which wiped out about half of the European population.

While the consequences and impacts of plague pandemics are well-documented, historians can't agree on how and when the bacteria emerged and started sickening humans.

Until this newest study, the earliest molecular evidence we had of a plague pandemic dates back 1,500 years to the Plague of the Justinian in about 550 AD. Even though they didn't have the DNA evidence, scholars believed that the bacteria may have caused outbreaks as far as 2,500 years ago.

Now, new molecular evidence suggests that Yersinia pestis may have infected people even earlier than that, a whopping 5,000 years ago, researchers reported Oct. 22 in the journal Cell. This is nearly three times earlier than we originally believed.

The team came to this conclusion by examining DNA extracted from 101 human teeth aged 2,800 to 5,000 years from Asia and Europe. After combing the DNA sequence for bacterial fragments belonging to Yersinia pestis, which would tell the researchers that that person had been infected, they found that seven of the samples contained plague DNA. Thus, evidence that the bacteria existed as far as 5,000 years ago.

Courtesy of London Crossrail "This study changes our view of when and how plague influenced human populations and opens new avenues for studying the evolution of diseases," study author Eske Willerslev, of the University of Copenhagen, said in a press release.

The researchers note, however, that while this early form of the bacteria had the genes necessary to be deadly in humans, it did not yet contain the genetic material that would allow it to survive in fleas. This would come about 1,000 years later, the researchers report, and would suggest that this early version of the bacteria would not be virulent enough to completely demolish populations as it did in later pandemics.

The most common form, bubonic plague, is generally transmitted from the bite of an infected flea and can cause a sudden fever, headache, chills, weakness, and large, swollen, and painful lymph nodes. If not treated quickly with antibiotics, the infection can spread to the rest of the body and become fatal within days.

But this new evidence that the bacteria lived as long as 5,000 years ago may explain certain earlier disease outbreaks that have mystified researchers. It also could have shaped the previously unexplained migration patterns of ancient human populations.

"Perhaps people were migrating to get away from epidemics or re-colonizing new areas where epidemics had decimated the local populations," study author Morten Allentoft of the University of Copenhagen said in a press release. "Could it be, for example, that plague was present in humans already in these prehistoric times?"